The present invention relates to a semiconductor design technology, and more particularly, to a semiconductor memory device using a magnetic tunnel junction device (MTJ).
In general, a dynamic random access memory (DRAM) device and a static random access memory (SRAM) device are a volatile memory device that loses data stored in a memory cell when power is not applied thereto. Therefore, many researches have been made to develop a non-volatile memory device. Among newly developed memory devices, a magnetic random access memory (MRAM) has been receiving attention as the next generation semiconductor device because the MRAM device not only has non-volatile characteristics but also has high integration, high speed operation, and low power consumption characteristics.
A memory cell of the MRAM device includes a transistor for performing a switching operation corresponding to an address applied from an external device and a magnetic tunnel junction device (MTJ) for storing information. The magnetic tunnel junction device (MTJ) is a kind of a magnetic memory device and has a magnet-to-resistance (MR) ratio that varies according to a magnetization direction of two ferromagnets. The MRAM device determines whether data stored in the magnetic tunnel junction device is ‘1’ or ‘0’ by sensing current according to the variation of the MR ratio.
FIG. 1 is a diagram illustrating a memory cell of a semiconductor memory device according to the prior art.
Referring to FIG. 1, the memory cell includes a NMOS transistor 110 and a MTJ device 130.
The NMOS transistor 110 includes a source-drain path formed between a source line SL and the MTJ device 130 and a gate connected to a word line WL. Such a NMOS transistor 110 is turned on/off according to activation of the word line WL. The world line is selected by a row address.
The MTJ device 130 includes a free layer 132, a tunnel insulating layer 134, and a pinned layer 136. The free layer 132 is formed of ferromagnetic substance and has a magnetization direction varying according to external stimulation, for example, current penetrating the MTJ 130. The pinned layer 136 has a magnetization direction that is not changed although an external stimulation is applied thereof. Particularly, the pinned layer 136 has a magnetization direction fixed by a pinning layer (not shown) formed of antiferromagentic, and the tunnel insulating layer 134 may be formed of magnesium oxide MgO.
Tunneling current flows in the magnetic tunnel junction device 130 according to a voltage applied to the both ends thereof. When the magnetization direction of the free layer 132 is matched with the magnetization direction of the pinned layer 136, the resistance value of the magnetic tunnel junction device 130 becomes small. When the magnetization direction of the free layer 132 is not matched with the magnetization direction of the pinned layer 136, the resistance value of the magnetic tunnel junction device 130 becomes large. In general, if the magnetization direction of the free layer 132 is identical to that of the pinned layer 136, the magnetic tunnel junction denotes denotes ‘1’. If not, the magnetic tunnel junction denotes ‘0’.
In other word, when positive current higher than a threshold current flows into the free layer 132 by applying a positive voltage to the free layer 132, which is a predetermined level higher than that applied to the pinned layer 136, the magnetization directions of the free layer 132 and the pinned layer 136 become identical. That is, a write operation for writing ‘0’ is performed and a resistance value of the magnetic tunnel junction device 130 becomes small. On the contrary, when negative current higher than a threshold current flows into the free layer 132 by applying a negative voltage to the free layer 132, which is a predetermined level higher than that of the pinned layer 136, the magnetization directions of the free layer 132 and the pinned layer 136 become opposite. That is, a write operation for writing ‘1’ is performed and a resistance value of the magnetic tunnel junction device 130 becomes large.
FIG. 2 is a graph showing tunnel magnet-to-resistance (TMR) characteristics according to a temperature of a magnetic tunnel junction device 130 of FIG. 1.
As shown in FIG. 2, the magnetic tunnel junction device 130 has hysteresis and two stable states according to a positive current or a negative current higher than a threshold voltage, that is, a state having small resistance value and a state having a large resistance value. The stable states are continuously sustained although the power is not applied.
As shown in FIG. 2, the magnetic tunnel junction device 130 has a resistance value that changes according to a temperature. Particularly, if the magnetization directions are opposite to each other and a temperature increases, a resistance value decreases. That is, the tunnel magnet-to-resistance characteristics change according to a temperature. The tunnel magnet-to-resistance characteristics make resistance value difference of data ‘1’ and ‘0’ gradually smaller. Therefore, it makes a semiconductor memory device difficult to determine whether the magnetic tunnel junction device 130 sustains a small resistance value or a large resistance value. Such a problem causes a semiconductor memory device not to properly read stored data when a semiconductor memory device performs a read operation.